Methods and apparatuses for forming optical preforms from glass soot

ABSTRACT

Methods and apparatuses for forming optical preforms from silica glass soot are disclosed. According to one embodiment, a method for forming an optical preform may include loading silica glass soot in a mold cavity of a mold body. The mold body may be rotated at a rotational speed sufficient to force the silica glass soot towards an inner wall of the mold body. Thereafter the silica glass soot is compressed in an inward radial direction as the mold body is rotated to form a soot compact layer.

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/904,085 filed on Nov. 14, 2013,the content of which is relied upon and incorporated herein by referencein its entirety.

BACKGROUND

1. Field

The present specification generally relates to the manufacture ofoptical preforms, such as optical fiber preforms and, more specifically,to methods and apparatuses for forming optical preforms from glass soot.

2. Technical Background

Conventionally, the outer cladding portion of an optical fiber preformmay be formed from an outside vapor deposition (OVD) process in whichsilica glass is deposited on a glass core blank, for example, throughthe pyrolysis of octamethyltetrasiloxane. The OVD process is a highlyoptimized, high yield manufacturing process. However, the formation ofthe cladding layer is often the rate limiting step in maximizing opticalfiber output. Further, it is estimated that as little as 50% of thepyrolysis product of the octamethyltetrasiloxane feedstock is depositedon the glass core blanks during deposition of the outer cladding portionof the optical fiber preform. The remaining pyrolysis product of theoctamethyltetrasiloxane feedstock is collected in a baghouse as silicaglass soot. The collected silica glass soot has a relatively highpurity, but still has contaminants which prevent it from being used toproduce high quality claddings of optical fiber preforms. Attempts havebeen made to use a soot pressing process with this waste silica byemploying a strenuous chemistry process to remove the impurities withmixed results. As a consequence, high purity soot generation processeshave been developed on an industrial and lab scale permitting the sootcharacteristics (such as particle distribution and the index ofrefraction by the addition of dopants) to be altered at will.

Prior techniques for forming cladding layers of an optical preform fromsilica glass soot, such as vertical pressing, are only suitable forforming a single cladding layer. As such, these techniques cannot beutilized to form preforms that have a complex refractive index profilethrough the radial cross section of the preform. Specifically, thesetechniques cannot be used to reliably construct an optical preform whichincludes multiple layers of silica glass soot where each layer is formedfrom silica glass soot with a different composition and/or morphology.

Accordingly, a need exists for alternative methods and apparatuses forforming an optical preform, such as an optical fiber preform, fromsilica glass soot.

SUMMARY

According to one embodiment, a method for forming an optical preform mayinclude loading silica glass soot in a mold cavity of a mold body. Themold body may be rotated at a rotational speed sufficient to force thesilica glass soot towards an inner wall of the mold body. Thereafter thesilica glass soot is compressed in an inward radial direction as themold body is rotated to form a soot compact layer.

In another embodiment, a method for forming an optical preform mayinclude loading a first amount of silica glass soot in a mold cavity ofa mold body. The mold body may be rotated at a rotational speedsufficient to force the first amount of silica glass soot towards aninner wall of the mold body. The first amount of silica glass soot maybe compressed in an inward radial direction as the mold body is rotatedto form a first soot compact layer. A second amount of silica glass sootmay be loaded in the mold cavity of the mold body around the first sootcompact layer. The mold body may be rotated at a rotational speedsufficient to force the second amount of silica glass towards the innerwall of the mold body. The second amount of silica glass soot may becompressed in an inward radial direction as the mold body is rotated toform a second soot compact layer around the first soot compact layer.

In yet another embodiment, an apparatus for forming an optical preformmay include a mold body comprising a mold cavity and an elasticallydeformable bladder. The elastically deformable bladder may be disposedwithin and lines the mold cavity adjacent to an inner wall of the moldbody. A fluid source may be coupled to the mold cavity between the innerwall of the mold body and the elastically deformable bladder. The fluidsource may supply a compressing fluid to the mold cavity to compress theelastically deformable bladder in a radially inward direction. A rotarydevice may be attached to at least one of a first end of the mold bodyand a second end of the mold body, the rotary device rotating the moldbody about a rotational axis of the mold body.

Additional features and advantages of the methods described herein willbe set forth in the detailed description which follows, and in part willbe readily apparent to those skilled in the art from that description orrecognized by practicing the embodiments described herein, including thedetailed description which follows, the claims, as well as the appendeddrawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a soot pressing apparatus for forming anoptical preform from silica glass soot according to one or moreembodiments shown and described herein;

FIG. 2 schematically depicts a cross section of the mold of the sootpressing apparatus of FIG. 1, according one or more embodiments shownand described herein;

FIG. 3 schematically depicts a cross section of the mold of the sootpressing apparatus as the mold body is being loaded with silica glasssoot;

FIG. 4 schematically depicts a cross section of the soot pressingapparatus of FIG. 1 with silica glass soot in the mold cavity as themold is being rotated;

FIG. 5 schematically depicts a cross section of the soot pressingapparatus of FIG. 1 with silica glass soot in the mold cavity as themold is being rotated and the soot is being compressed radially inward;and

FIG. 6 schematically depicts a cross section of the mold of FIG. 2 as asecond amount of silica glass soot is loaded around a preform compactassembly.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of methods andapparatuses for forming optical preforms from silica glass soot,examples of which are illustrated in the accompanying drawings. Wheneverpossible, the same reference numerals will be used throughout thedrawings to refer to the same or like parts. In one embodiment, a methodfor forming an optical preform may include loading silica glass soot ina mold cavity of a mold body. The mold body may be rotated at arotational speed sufficient to force the silica glass soot towards aninner wall of the mold body. Thereafter the silica glass soot iscompressed in an inward radial direction as the mold body is rotated toform a soot compact layer. Various embodiments of methods andapparatuses for forming optical preforms from silica glass soot will bedescribed in more detail herein with specific reference to the appendedfigures.

The following terminology will be used herein to described the sootpreforms and optical fiber preforms formed therefrom:

The term “refractive index profile,” as used herein, is the relationshipbetween the refractive index and the radius of the preform compactassembly and/or optical preform.

The term “up-dopant,” as used herein, refers to a dopant which raisesthe refractive index of silica glass relative to pure, undoped SiO₂. Theterm “down-dopant,” as used herein, is a dopant which lowers therefractive index of silica glass relative to pure, undoped SiO₂.

The term “substantially free,” as used herein with reference to silicaglass soot, means that the silica glass soot contains less than 0.1 wt.% of a specified material as either a contaminant or tramp constituent(i.e., the material is not intentionally added to the silica glasssoot).

Referring now to FIG. 1, a soot pressing apparatus 100 for forming anoptical preform from silica glass soot is schematically depicted. Thesoot pressing apparatus 100 includes a mold 102 which is rotationallycoupled to a rotary device 104 for rotating the mold 102 about a longaxis 106 of the mold 102 (i.e., the axis of rotation of mold 102). Therotary device 104 may be any suitable device for imparting rotation tothe mold 102. For example, in the embodiment of the soot pressingapparatus 100 depicted in FIG. 1, the rotary device 104 is a lathe. Inan alternative embodiment (not shown), the rotary device may be a rollertable comprising one or more actively driven rollers which engage withthe mold and rotate the mold about the long axis of the mold. However,it should be understood that other devices suitable for impartingrotation to the mold may be used including, without limitation,purpose-built devices.

Referring now to FIGS. 1 and 2, FIG. 2 schematically depicts a crosssection of the mold 102 of the soot pressing apparatus 100 of FIG. 1.The mold 102 generally comprises a mold body 108 having a mold cavity110 formed therein. The mold cavity 110 is circular in radial crosssection (i.e., the section through the mold in the +/−y-direction or the+/−x-direction of the coordinate axes depicted in FIG. 2). Accordingly,it should be understood that the mold cavity is generally cylindrical.The mold body 108 may be formed from aluminum alloys, steel, carbonfiber, polymeric material or any other suitable material. In theembodiment shown in FIG. 2, the mold body is formed from an aluminumalloy.

The mold 102 further comprises an elastically deformable bladder 112positioned within the mold cavity 110 adjacent to an inner wall 114 ofthe mold body 108. The elastically deformable bladder 112 is formed froman elastically deformable and recoverable material such as latex rubber,silicone, neoprene, cast urethane or the like. In the embodimentsdescribed herein, the elastically deformable bladder 112 is formed fromlatex rubber having a thickness from about 15 thousandths of an inch toabout 30 thousandths of an inch (˜0.3 mm to about 0.8 mm). However, itshould be understood that other thicknesses for the elasticallydeformable bladder 112 are contemplated. It should also be understoodthat other materials for the elastically deformable bladder 112 are alsocontemplated.

The elastically deformable bladder 112 is generally in the shape of acylindrical sleeve which is open at both ends. In embodiments, thediameter of the elastically deformable bladder 112 is less than theinner diameter D_(I) of the mold cavity 110. The elastically deformablebladder 112 is positioned in the mold cavity 110 and the ends of thebladder are stretched and folded over the ends of the mold body 108. Thefolded ends are secured in place on the outer diameter of the mold body108 with elastic bands 120 around which hose clamps 122 are positioned.When in a relaxed state, the elastically deformable bladder 112 is atleast partially spaced apart from the inner wall 114 of the mold cavity110, as depicted in FIG. 2.

The mold 102 further comprises a lower end cap 124 and an upper end cap128 affixed to opposite ends of the mold body 108 (i.e., the first endand the second end of the mold body 108 in the +/−z-direction of thecoordinate axes of FIG. 2) such that the lower end cap 124 and the upperend cap 128 are opposed to one another in the direction of the long axis106 of the mold body 108. The lower end cap 124 and the upper end cap128 may be formed from aluminum alloys, steel, carbon fiber, polymericmaterial or any other suitable material. In the embodiments describedherein, the lower end cap 124 and the upper end cap 128 are formed fromthe same material as the mold body 108 (i.e., an aluminum alloy).However, it should be understood that, in other embodiments, the lowerend cap 124 and the upper end cap 128 may be formed from material whichis different than that of the mold body 108. In the embodimentsdescribed herein, the lower end cap 124 and the upper end cap 128 havediameters which are greater than the outer diameter of the mold body 108such that both the upper end cap 128 and the lower end cap 124 extendbeyond the outer diameter of the mold body and form flanges in the+/−y-direction and the +/−x-direction.

Still referring to FIG. 2, in embodiments where the soot pressingapparatus 100 is used to form an optical fiber preform comprising a sootcompact pressed around a central glass core blank, at least one of thelower end cap 124 and the upper end cap 128 may include a blind notch126 for receiving an end of a glass core blank when the glass core blankis positioned within the mold cavity 110. In the embodiment of the mold102 depicted in FIG. 2, the blind notch 126 is formed in the lower endcap 124. In this embodiment, the blind notch 126 is a v-shaped notchformed in the lower end cap 124. However, it should be understood thatother configurations and geometries of the blind notch are contemplated.The blind notch 126 is formed in the lower end cap 124 such that thelong axis of the glass core blank can be aligned with the long axis ofthe mold 102 when the glass core blank is engaged with the blind notch126.

Similarly, in embodiments where the soot pressing apparatus 100 is usedto form an optical fiber preform comprising a soot compact pressedaround a central glass core blank, at least one of the lower end cap 124and the upper end cap 128 may include an aperture 144 for receiving anend of a glass core blank when the glass core blank is positioned withinthe mold cavity 110. In the embodiment of the mold 102 depicted in FIG.2, the aperture 144 is formed in the upper end cap 128. The aperture 144is formed in the upper end cap 128 such that the long-axis of the glasscore blank is substantially co-linear with the long axis of the mold 102when the glass core blank is engaged with the blind notch 126 andpositioned in the aperture 144.

In embodiments, the lower end cap 124 and the upper end cap 128 may besecured to the mold body 108 with threaded rods 136, 138 (FIG. 1) whichextend through the end caps 124, 128 and are secured with threadedfasteners, such as lock nuts or the like. In embodiments, the lower endcap 124 and the upper end cap 128 may be formed with a mating groove(not shown) in which the ends of the mold body 108 are positioned tofacilitate centering the end caps 124, 128 on the mold body 108.

The mold 102 may further comprise an adapter 140 (QF adapter) alignedwith the aperture 144 formed in one of the lower end cap 124 or theupper end cap 128. The adapter 140 facilitates coupling an adapter tube142 (such as a QF40 tube) to the aperture 144. The adapter tube 142 maybe used to assist in centering and stabilizing a glass core blank 170 inthe mold cavity 110 during a soot pressing operation. To further assistin centering and stabilizing a glass core blank in the mold cavity 110,one or more compression springs 143 may be disposed in the interior ofthe adapter tube 142. The upper end of the springs may be held in placewith a stop (not shown), such as a QF40-to-Swagelok® adapter. The lowerend of the springs may seat on a washer (not shown) through which theglass core blank 170 is inserted. The adapter tube 142 may optionally beused to draw a vacuum on the mold cavity 110 during a soot pressingoperation, thereby evacuating air from the mold cavity as silica glasssoot is compressed, as will be described in further detail herein.

Still referring to FIG. 2, the mold 102 may further comprise a pair ofstopper cushions 116, 118 which are positioned in the mold cavity 110 atopposite ends of the mold body 108. In embodiments where the sootpressing apparatus 100 is used to form a soot compact layer pressedaround a central glass core blank, the stopper cushions 116, 118 mayhave an annular shape with a central annulus for receiving a glass coreblank. The stopper cushions 116, 118 are formed from a compressible,open-cell foam material, such as polyurethane foam or a similarmaterial. The stopper cushions 116, 118 protect and cushion theelastically deformable bladder 112 and the preform compact layer formedin the mold cavity 110. The open-cell structure of the stopper cushionsalso allows air to escape from the interior of the mold cavity 110,either passively or actively (such as when a vacuum is drawn on the moldcavity 110), as the elastically deformable bladder is compressedradially inward, as will be described in further detail herein.

The mold 102 may further include a fitting 130 coupled through the moldbody 108 such that fluid, such as air or the like, may be pumped into orevacuated from the mold cavity between the inner wall 114 of the moldbody 108 and the elastically deformable bladder 112. In the embodimentof the mold 102 depicted in FIG. 2, the fitting 130 is coupled throughthe sidewall of the mold body 108 and fluidly coupled to a supplychannel 132 formed in the lower end cap 124. The inlet of the supplychannel 132 is positioned in the lower end cap 124 on the long axis ofthe mold 102 and may be coupled to a rotary union 134. The orientationof the inlet of the supply channel 132 on the long axis of the mold 102and the use of a rotary union 134 permit fluid to be pumped into and/orevacuated from the mold cavity 110 as the mold 102 is rotated about itslong axis 106.

While embodiments of the soot pressing apparatus 100 described hereinare constructed to accommodate a glass core blank in order to form oneor more soot compact layers around the glass core blank, it should beunderstood that alternative constructions are possible and contemplated.For example, the soot pressing apparatus described herein may also beconstructed to form an optical preform without a central core blank. Inthese embodiments (not shown) the lower end cap 124 may be formedwithout a blind notch. Similarly, the stopper cushions 116, 118 may beformed without a central annulus. Forming an optical preform without acentral core blank may also eliminate the need for the at least onecompression spring 143 in the adapter tube 142.

Referring now to FIG. 3, to form an optical fiber preform with the sootpressing apparatus, a glass core blank 170 is positioned in the moldcavity 110 of the mold body 108 such that an end of the glass core blank170 is engaged with the blind notch 126. A first amount of silica glasssoot 172 is then loaded into the mold cavity 110 through the upper endcap 128. This first amount of silica glass soot 172, which ultimatelyforms a first preform cladding layer around the glass core blank 170,may have a first composition and/or a first morphology. Regarding thecomposition of the silica glass soot, the soot may comprise silica glasswhich is substantially free from any dopant materials such as up-dopantsor down-dopants. Alternatively, the silica glass soot may comprisesilica glass soot with one or more dopants which either increase and/ordecrease the index of refraction of silica glass. For example, suitableup-dopants for increasing the index of refraction of the silica glasssoot may include, without limitation, GeO₂, Al₂O₃, P₂O₅, TiO₂, ZrO₂,Nb₂O₅, Ta₂O₅, Cl and/or combinations thereof. Suitable down-dopants fordecreasing the index of refraction of the silica glass soot may include,without limitation, F, B₂O₃, SiF₄, CF₄, C₂F₆, and/or combinationsthereof. Silica glass soot having the appropriate dopant(s) (or lackthereof) may be selected to achieve a specific refractive index profilein the optical fiber preform resulting from the soot compact assemblyformed from the soot pressing apparatus 100. Regarding the morphology ofthe silica glass soot, silica glass soot having a desired morphology(i.e., the size and shape of the individual particles, particle sizedistribution, etc.) may be added to achieve a preform cladding layerhaving a desired porosity, tortuosity, density, surface area, or thelike following the pressing operation.

Vibratory energy may be applied to the mold body 108 to aid in settlingthe soot in the mold cavity 110 and to remove any pockets of air thatare trapped in the silica glass soot. This vibratory energy may beapplied as the silica glass soot is loaded into the mold cavity 110and/or after the silica glass soot is loaded into the mold cavity. Inone embodiment, the vibratory energy may be applied to the mold body 108by positioning the mold body on a vibrating plate 200, as depicted inFIG. 3. Alternatively, the vibratory energy may be coupled into the moldbody 108 using an apparatus as described in U.S. patent application Ser.No. 12/603,960 filed Oct. 22, 2009 entitled “Methods for FormingCladding Portions of Optical Fiber Preform Assemblies” and assigned toCorning Inc.

As the silica glass soot 172 is loaded into the mold cavity 110, avacuum may be optionally drawn on the mold cavity 110 through thefitting 130 using vacuum pump 202. Drawing a vacuum on the mold cavitythrough the fitting 130 evacuates the air between the elasticallydeformable bladder 112 and the inner wall 114 of the mold cavity 110 andcauses the elastically deformable bladder 112 to elastically expandradially outward, towards the inner wall 114 of the mold cavity 110. Theoutward expansion of the elastically deformable bladder 112 allows for agreater volume of soot to be loaded into the mold cavity 110.

Referring now to FIG. 4, once the silica glass soot 172 is loaded intothe elastically deformable bladder 112 of the mold cavity 110, the upperstopper cushion (i.e., stopper cushion 116) is positioned in the moldcavity 110 around the glass core blank 170 and over the silica glasssoot 172. Thereafter, the adapter 140 is positioned around the glasscore blank 170 and affixed to the upper end cap 128 using threadedfasteners, such as bolts or the like. At least one compression spring143 is then disposed around the glass core blank 170 adjacent to ball171 formed on the glass core blank 170. The adapter tube 142 is thenpositioned around the glass core blank 170 and the at least onecompression spring 143 and inserted into the adapter 140 where it issecured in place. As the adapter tube 142 is inserted into the adapter140, the adapter tube 142 engages with the at least one compressionspring 143, compressing the at least one compression spring 143 and bothcentering and stabilizing the glass core blank 170 in the mold body 108.The interaction between the glass core blank 170, the at least onecompression spring 143, and the adapter tube 142 prevents the glass coreblank 170 from wobbling in the mold cavity 110 as the mold 102 issubsequently rotated.

In some embodiments described herein, loading the mold 102 with silicaglass soot and securing the glass core blank 170 in the mold 102 isperformed while the mold 102 is substantially vertically oriented (i.e.,the long axis of the mold is substantially parallel with the+/−z-direction of the coordinate axes depicted in FIG. 2.

Once the mold 102 is loaded with silica glass soot 172 and the glasscore blank 170 is secured and stabilized within the mold cavity 110 ofthe mold body 108, the mold 102 is coupled to the rotary device 104. Inembodiments where the rotary device 104 is a lathe, as depicted in FIG.4, the mold 102 is clamped into the chucks 150, 152 of the lathe tosecure the mold 102 for rotation by the lathe. In the embodimentsdescribed herein, the mold 102 is clamped in the chucks 150, 152 withlathe dawgs 154 such that mold 102 can be rotated about the long axis106 of the mold 102. In addition, the mold 102 is clamped in the chucks150, 152 such that the long axis 106 of the mold is substantiallyhorizontal (i.e., parallel with the +/−y-direction of the coordinateaxes depicted in FIG. 4). This allows the silica glass soot to beuniformly distributed along the length of the mold 102 (i.e., thedimension in the +/−y-direction). Thereafter, the rotary union 134 isconnected to the fitting 130 through the lower end cap 124. The rotaryunion 134 is then coupled to a fluid source 300 which, in thisembodiment, is a compressor. Alternatively, the fluid source may be acompressed gas cylinder or a similar source of compressed fluid.Optionally, a second rotary union 135 may be coupled to the adapter tube142 to facilitate coupling a vacuum pump 305 to the adapter tube toevacuate air from the interior of the mold cavity 110, specifically theinterior of the elastically deformable bladder 112, as the mold 102 isrotated with the rotary device.

Still referring to FIG. 4, the mold 102 is then rotated about the longaxis 106 with the rotary device 104 at a rotational speed sufficient toforce the silica glass soot 172 radially outwards, towards the innerwall 114 of the mold body, as depicted in FIG. 4. This force imparted tothe silica glass soot 172 by the rotation of the mold 102 causes thesilica glass soot to become evenly distributed over the interior of theelastically deformable bladder 112, thereby forming a layer of soot witha substantially uniform thickness. In order to achieve this evendistribution of soot over the interior of the elastically deformablebladder 112, the mold 102 is rotated at speed from greater than or equalto about 50 RPM to less than or equal to about 500 RPM. For example, inone embodiment, the mold 102 may be rotated at a speed from greater thanor equal to about 100 RPM to less than or equal to about 400 RPM. Inanother embodiment, the mold 102 may be rotated at a speed from greaterthan or equal to about 75 RPM to less than or equal to about 115 RPM.

Referring now to FIG. 5, as the mold 102 is rotated with the rotarydevice 104, a compression fluid, such as air, compressed gas, or thelike, is introduced into the mold cavity 110 between the inner wall 114of the mold body and the elastically deformable bladder 112. Thecompression fluid is directed into the mold cavity from a fluid sourcecoupled to the rotary union 134. As the compression fluid enters themold cavity 110, the compression fluid acts on the elasticallydeformable bladder 112, displacing the elastically deformable bladder112 radially inward which, simultaneously, compresses the silica glasssoot in a radial inward direction, toward the glass core blank 170,thereby forming a preform soot compact layer 174 around the glass coreblank 170.

In some embodiments described herein, the compression fluid may beintroduced into the mold cavity 110 at a controlled ramp rate. Forexample, in one embodiment, the compression fluid may be introduced intothe mold cavity such that the pressure in the mold cavity 110 betweenthe inner wall 114 and the elastically deformable bladder 112 increasesat a rate from about 1 psi/min (about 6 kPa/min) to about 10 psi/minute(about 60 kPa/min). In embodiments, the compression fluid may beintroduced into the mold cavity 110 until the pressure in the moldcavity 110 between the inner wall 114 and the elastically deformablebladder 112 reaches a maximum value of less than or equal to about 200psi (about 1.5 MPa). In some embodiments, the maximum pressure issufficient to compress the silica glass soot to a density from about 0.5g/cc to about 0.9 g/cc. While specific values for the maximum pressureand ramp rate have been described herein, it should be understood thatother values are contemplated and that the specific pressure values(i.e., ramp rate and maximum pressure) are selected to achieve aspecific density for each preform soot compact layer 174 formed aroundthe glass core blank 170.

As the silica glass soot 172 is compressed around the glass core blank170, air trapped in the silica glass soot 172 is forced out of the moldcavity through the stopper cushion 116 and the adapter tube 142. In someembodiments, vacuum pump 305 may be coupled to the adapter tube 142 witha rotary union 135, as described above. As the silica glass soot 172 iscompressed, a vacuum is drawn on the adapter tube 142 to assist with theremoval of air from the silica glass soot 172. However, because theelastically deformable bladder 112 separates and seals the interior ofthe elastically deformable bladder 12 (and the silica glass soot 172)from the inner wall 114 of the mold body 108, drawing a vacuum throughthe adapter tube 142 does not adversely impact the compression of thesilica glass soot due to the introduction of compression fluid betweenthe inner wall 114 of the mold body 102 and the elastically deformablebladder 112. Indeed, actively removing air from the silica glass soot172 as the silica glass soot is compressed may actually expedite thecompaction process.

Once the maximum pressure between the inner wall 114 and the elasticallydeformable bladder 112 has been reached, the pressure in the mold cavityand the rotational speed of the mold 102 are held constant for apredetermined dwell time to ensure that the silica glass soot isadequately compressed. Thereafter, the rotational speed of the mold 102and the pressure in the mold cavity 110 are gradually decreased toensure the integrity of the preform soot compact layer 174 formed aroundthe glass core blank 170 which, collectively, form a preform compactassembly 180.

After the first preform soot compact layer 174 is formed around theglass core blank 170, one or more additional preform soot compact layersmay be formed around the first preform soot compact layer 174 to achievea specific refractive index profile in the preform compact assembly 180.Referring to FIG. 6 by way of example, after the first preform sootcompact layer 174 is formed, the mold 102 is removed from the rotarydevice 104 and the adapter tube 142, adapter 140, and the at least onecompression spring 143 are removed from around the glass core blank 170.Thereafter, a second amount of silica glass soot 176 is loaded into themold cavity 110 in the elastically deformable bladder 112 and around thefirst preform soot compact layer 174. In some embodiments, thecomposition and/or morphology of the second amount of silica glass soot176 may be the same as the first amount of silica glass soot forming thefirst preform soot compact layer 174. In some other embodiments, thecomposition and/or morphology of the second amount of silica glass soot176 may be different than the first amount of silica glass soot formingthe first preform soot compact layer 174. As described hereinabove,vibratory energy may be applied to the mold body 108 as the secondamount of silica glass soot 176 is loaded into the mold cavity 110.Also, a vacuum may be optionally drawn on the mold cavity 110 throughthe fitting 130, as described above, to expand the elasticallydeformable bladder 112 radially outward, towards the inner wall of themold cavity 110 as the second amount of silica glass soot 176 is loadedinto the mold cavity 110.

Thereafter, the stopper cushion 116, adapter 140, at least onecompression spring 143 and adapter tube 142 are reassembled around theglass core blank 170, as described above. The mold 102 is thenreinserted in the rotary device 104, and the process of rotating themold 102 as the silica glass soot is radially compressed is repeated toform a second preform soot compact layer around the first preform sootcompact layer. This process (i.e., load silica glass soot, rotate moldwhile simultaneously compressing silica glass soot) may be repeated anynumber of times to achieve a preform compact assembly with the desiredrefractive index profile, soot layer thicknesses, and soot layerdensities.

The resultant preform compact assembly comprising one or more preformsoot compact layers formed around a glass core blank may then beconsolidated to sinter the preform soot compact layers thereby forming adense silica glass cladding portion around the glass core blank, thusproducing an optical preform, such as an optical fiber preform. Theconsolidation of the preform soot compact layers also joins the claddingportion of the preform to the glass core blank 170 thereby forming anoptical preform, such as an optical fiber preform.

In one embodiment, the preform compact assembly having one or morepreform soot compact layers formed around a glass core blank may beconsolidated to an optical fiber preform by affixing a handle to theglass core blank and hanging the preform soot compact assembly from aquartz immersion rod over a consolidation furnace. The consolidationfurnace may generally comprise a tube furnace with a quartz mufflehaving a drying zone and a consolidation zone. The drying zone may beheld at a temperature of about 1000° C. while the consolidation zone hasa temperature gradient from about 1000° C. to about 1450° C. across thezone. The consolidation zone of the consolidation furnace may bemaintained under a helium flow. The preform compact assembly is held inthe drying zone of the consolidation furnace and successively exposed toa flow of helium and oxygen and a flow of helium and chlorine in twoisothermal hold periods in order to dry the preform compact assembly andremove carbon, water and transition metal impurities. After the dryingtreatment, the atmosphere in the tube furnace is then switched to ahelium flow and the preform compact assembly is lowered through theconsolidation zone to increase the temperature of the silica glass sootcreating a vitreous flow of glass sufficient to form fully consolidatedglass.

While a conventional consolidation of the preform compact assembly hasbeen described herein above, it should be understood that alternativeconsolidation processes are contemplated including, without limitation,low pressure consolidation.

Following consolidation, the consolidated preform compact assembly (nowoptical fiber preform) is withdrawn from the consolidation furnace andloaded into a 1000° C. holding oven for at least six hours to de-gas andanneal the preform. Thereafter, the optical fiber may be drawn from thepreform.

EXAMPLES

The embodiments described herein will be further clarified by thefollowing example.

Example 1

A preform compact assembly having a glass core blank surrounded by threesoot compact layers was formed. The preform compact assembly was formedby positioning the glass core blank within the elastically deformablebladder of the mold apparatus as depicted in FIG. 2. The glass coreblank consisted of a 12 mm diameter glass core blank with an additionallayer of outside vapor deposited (OVD) silica such that the totaldiameter of the glass core blank was 57 mm. Approximately 150 grams ofsilica glass soot was poured into the elastically deformable bladderaround the glass core blank.

To form the first soot compact layer around the glass core blank, themold apparatus was then rotated at 115 RPM to distribute the silicaglass soot evenly throughout the interior of the mold apparatus. As themold apparatus was rotated, the elastically deformable bladder wascompressed around the glass core blank by supplying compressed air tothe mold apparatus between the interior wall of the mold cavity and theelastically deformable bladder, thereby compressing the silica glasssoot around the glass core blank. The pressure applied to theelastically deformable bladder was ramped from 0 psi to 150 psi at aramp rate of 5 psi/min. The maximum pressure was maintained for a holdperiod of 15 minutes. Thereafter, the pressure was decreased to 0 psi ata ramp rate of −10 psi/min.

The second and third soot compact layers were formed around the firstsoot compact layer using a similar process. Specifically, for each ofthe second and third soot compact layers, approximately 150 grams ofsilica glass soot was poured into the elastically deformable bladderaround the previously formed soot compact layer. The mold apparatus wasthen rotated at approximately 75 RPM. Slower rotational speeds were usedin the formation of the second and third soot compact layers to preventdamage to previously formed soot compact layers. As with the first sootcompact layer, the second and third soot compact layers were formed bycompressing the silica glass soot around the glass core blank bysupplying compressed air to the mold apparatus between the interior wallof the mold cavity and the elastically deformable bladder, therebycompressing the silica glass soot around the glass core blank. Thepressure applied to the elastically deformable bladder was ramped from 0psi to 150 psi at a ramp rate of 5 psi/min. The maximum pressure wasmaintained for a hold period of 15 minutes. Thereafter, the pressure wasdecreased to 0 psi at a ramp rate of −10 psi/min.

The resulting preform compact assembly included a glass core blank witha diameter of 52 mm surrounded by three concentric soot compact layersformed from compressed silica glass soot. Each of the soot compactlayers had a radial thickness of approximately 0.25 inches.

Based on the foregoing, it should now be understood that the methods andapparatuses described herein may be used to form an optical preform,such as an optical fiber preform, by compressing silica glass soot in amold as the mold is rotated. The rotation of the mold allows the silicaglass soot to be evenly distributed in the mold, thereby improving theuniformity of the resultant preform compact assembly and, ultimately,the optical preform formed from the preform compact assembly.

The methods and apparatuses described herein enable the formation of anoptical preform from multiple layers of silica glass soot, wherein eachconsecutive layer is formed with silica glass soot having a differentcomposition and/or morphology. This allows the optical preform to beconstructed with a desired refractive index profile using consecutivesoot pressing operations.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method for forming an optical preform, themethod comprising: loading silica glass soot in a mold cavity of a moldbody; rotating the mold body at a rotational speed sufficient to forcethe silica glass soot towards an inner wall of the mold body; andcompressing the silica glass soot in an inward radial direction as themold body is rotated to form a soot compact layer.
 2. The method ofclaim 1, further comprising orienting the mold body such that a longaxis of the mold body is substantially horizontal prior to rotating themold body, wherein the mold body is rotated about the long axis of themold body.
 3. The method of claim 1, further comprising drawing a vacuumon the mold cavity as the mold body is rotated.
 4. The method of claim1, further comprising positioning a glass core blank in the mold cavityprior to loading the silica glass soot in the mold cavity, wherein thesilica glass soot is compressed in an inward radial direction around theglass core blank to form the soot compact layer.
 5. The method of claim1, wherein the mold body is rotated at a rotational speed from greaterthan or equal to about 50 RPM to less than or equal to about 500 RPM. 6.The method of claim 1, wherein the silica glass soot is compressed inthe inward radial direction at a ramp rate from about 1 psi per minuteto about 10 psi per minute.
 7. The method of claim 1, wherein the silicaglass soot is compressed with a maximum pressure less than or equal to200 psi.
 8. The method of claim 1, wherein the silica glass soot iscompressed to a density from about 0.5 g/cc to about 0.9 g/cc.
 9. Themethod of claim 1, wherein: the mold body comprises an elasticallydeformable bladder positioned in the mold cavity; the silica glass sootis loaded in the elastically deformable bladder; and the silica glasssoot is compressed in an inward radial direction by applying pressurebetween the mold body and the elastically deformable bladder.
 10. Themethod of claim 9, further comprising drawing a vacuum between the innerwall of the mold body and the elastically deformable bladder to expandthe elastically deformable bladder as the silica glass soot is loadedinto the elastically deformable bladder.
 11. A method for forming anoptical preform, the method comprising: loading a first amount of silicaglass soot in a mold cavity of a mold body; rotating the mold body at arotational speed sufficient to force the first amount of silica glasssoot towards an inner wall of the mold body; compressing the firstamount of silica glass soot in an inward radial direction as the moldbody is rotated to form a first soot compact layer; loading a secondamount of silica glass soot in the mold cavity of the mold body aroundthe first soot compact layer; rotating the mold body at a rotationalspeed sufficient to force the second amount of silica glass towards theinner wall of the mold body; and compressing the second amount of silicaglass soot in an inward radial direction as the mold body is rotated toform a second soot compact layer around the first soot compact layer.12. The method of claim 11, wherein a composition of the first amount ofsilica glass soot is different than a composition of the second amountof silica glass soot.
 13. The method of claim 11, wherein a morphologyof the first amount of silica glass soot is different than a morphologyof the second amount of silica glass soot.
 14. The method of claim 11,further comprising positioning a glass core blank in the mold cavityprior to loading the first amount of silica glass soot in the moldcavity, wherein the first amount of silica glass soot is compressed inan inward radial direction around the glass core blank to form the firstsoot compact layer.